Various types of battery chargers are known in the art. The two most common types of battery chargers are known as linear- and switched-mode battery chargers. Linear-mode battery chargers provide an output voltage that is a linear function of the input voltage. Unused charging power is simply dissipated. Switched-mode battery chargers are more efficient. With such switched-mode battery chargers, only slightly more than the input power required to generate the charging voltage and current is consumed to exist with such switched-mode battery chargers.
The charging characteristics of a battery charger are normally configured to match the battery chemistry of the battery to be charged. For example, lead acid batteries, frequently used in automotive and marine applications, are often charged with constant power, constant current or constant voltage or a combination thereof. Such batteries are known to be charged with both linear- as well as switched-mode battery chargers. U.S. Patent Application Publication No. US 2005/0088144 A1, assigned to the same assignee as the assignee of the present invention, discloses an example of a switched-mode battery charger for automotive and marine battery applications.
Many different considerations affect the selection of a particular battery chemistry for a particular application. For example, lead acid batteries are normally used in automotive and marine battery applications because of the ability to deliver relatively large amounts of power. In automotive applications, an initial burst of power is required to start the engine. In marine applications, such as U.S. Coast Guard applications, the battery capacity is an important consideration for use in buoys, deployed by the United States Coast Guard in the oceans surrounding the U.S. to transmit weather information to mariners. Such buoys are also used for navigation.
Battery capacity is normally measured in terms of ampere hours. Theoretically, the ampere hour capacity is the number of hours that the battery can deliver a specified level of output current. Due to losses within the battery, the ampere-hour output capability of a battery is known to be slightly less than the ampere-hour input.
During charging, it is necessary to charge the battery to its fully-charged condition without exceeding the voltage, current, or temperature, which may damage the battery, as specified by the battery manufacturer. An exemplary battery charging characteristic curve for an exemplary marine battery is illustrated in FIG. 1. In this example, the charging current is illustrated by the curve 20. The maximum charging current is limited by various parameters set forth by the battery manufacturer, such as temperature cut-off (TCO), the rate of change of temperature with respect to time (dT/dt), current, and other parameters. The battery charging temperature is identified with the curve 22. The curve 24 illustrates the battery voltage, while the curve 26 illustrates the ambient air temperature. In this particular example, the maximum charging current is limited to a value slightly greater than 40 amperes during a constant current mode during a time period t1. Based upon the charging characteristics illustrated in FIG. 1, the end of a nominal charging cycle is shown at the point 27. The ampere hours applied to the battery by the charger can be obtained by integrating the area under the curve 20. In this exemplary case, the ampere hours input to the battery is 40.65 Ah.
As mentioned above, due to internal losses within the battery, the output capacity of the battery will be slightly lower than 40.65 ampere hours. An exemplary discharge curve is illustrated in terms of FIG. 2. As shown, a fully charged exemplary battery in accordance with FIG. 1 is discharged at 10 amps while measuring the terminal voltage. For a nominal 12-volt automotive battery, the battery is discharged at 10 amps until the terminal voltage reached about 10.5 amps, which was about 235 minutes, as indicated by the point 29. The output capacity is thus 10 amperes×235 minutes×1 hour/60 minutes or about 39.2 ampere hours.
There are several problems with known chargers. First, the charging times are relatively long. Second, the charging characteristics of known battery chargers require such battery chargers to be rated at relatively low values.